The present invention relates to RF and microwave amplifiers.
These amplifiers can be embodied in technology of the traveling wave tube type or in semiconductor technology. They apply in particular to telecommunications or to radars.
An amplification stage taken in isolation necessarily has a gain which decreases onward of a certain input power, generally lower than the saturation power at which the amplification stage must operate in order to exhibit adequate efficiency. This results in distortions in the transmitted signal which themselves create intersymbol interference which is all the more problematic since the passband is used in a dense manner, this being the sought-after objective, in particular in telecommunications. Said distortions have the additional drawback of not being temperature-stable.
This is why devices are used to linearize a microwave amplification stage, their common objective being to correct the nonlinearities of the amplification stage in such a way as to ensure a constant gain at the amplification stage close to the saturation power and over the entire passband of said stage within a temperature range corresponding to the operating conditions of the equipment.
Three main types of linearizer devices are described in the state of the art.
The first type consists of feedback loop linearizer devices, in which the amplitude and phase parameters of the output signal from the amplification stage are measured so as to deduce therefrom the corrections to be applied to the input signal to obtain a linear output signal. (See in particular patents U.S. Pat. Nos. 4,291,277, 5,598,127, 5,422,598, 5,469,114, 5,524,285 and 5,722,056). Given the lag introduced by the feedback loop, these devices are not usable for passbands above 100 MHz.
The second type consists of open loop linearizer devices in which various digital processing techniques are applied to the amplitude, phase or pulse components of the signal so as to correct the distortions. (See in particular U.S. Pat. Nos. 5,760,646, 5,877,653 and 5,886,573). These devices presuppose recourse to complex digital signal processing techniques and the use of specific circuits matched to the waveform emitted. This path is not practical nowadays because of the limits of semiconductor technology for carrier frequencies above a few gigahertz.
The third type consists of predistortion linearizer devices in which the input signal is subjected before being introduced into the amplification stage to a gain distortion which increases with input power and which will compensate as precisely as possible for the symmetric distortion of the amplification stage.
The present invention belongs to this third type. (As do U.S. Pat. Nos. 4,878,030, 5,146,177, 5,291,148, 5,523,716, 5,576,660 and 5,736,898, the latter patent of Kohl et al constituting the reference state of the art of the present invention).
In Kohl et al, the basic diagram of which is illustrated in FIG. 1 hereinbelow, the input signal (10) is submitted to a separator (20) consisting of two identical Schottky diodes (22 and 23), arranged in parallel between two coupler circuits (21 and 24), the parameters of the diodes being chosen so that the separation device allows only the low-amplitude signals through to a so-called compression path (30), the high-amplitude signals being reflected toward another path, the so-called expansion path (40), connected to one of the branches of the input coupler so as to be presented to an amplifier (50), the compression path and the expansion path being recombined so as to be presented to the input of the amplification stage (90) by a combiner coupler (80). The paths 30 and 40 are furnished with phase control devices (60 and 70).
This device has the theoretical advantage of achieving automatic control of the phase of the reflected signal. Now, this theoretical advantage is difficult to obtain in practice because of the scatter in the manufacturing and operating parameters of the diodes. Moreover, when the separation device deviates from the balanced operating conditions obtained in the case of two identical diodes, the reflected signal will disturb the input signal with a reflection coefficient varying with input power.
An aim of the present invention is to correct these defects of the prior art.
For this purpose, the invention proposes a linearizer device for RF and microwave amplification stage of the type comprising an input, followed by a separator operating by level-dependent selective reflection, which forward-feeds a compression path, and by reflection an expansion path, while the outputs of these two paths are united by a combiner coupler so as to go to the amplification stage, characterized in that the separator comprises a circulator, whose input port is linked to the input of the device, in that a single level-dependent selective reflection arrangement is placed after the input/output port of the circulator, and followed by one of said paths, and in that the output port of the circulator is linked to the other path.
The device furthermore comprises on the compression path or on the expansion path a phase control circuit, adjusted as a function of a chosen phase compensation law, and an amplitude control circuit.
The device is also characterized by microwave line lengths on the compression and expansion paths which are substantially identical.
Finally the device can hinge around components operating in passive mode.